1. Field of the Invention
The invention relates to a system and a method for controlling an engine, and particularly, to a system and a method for controlling pressure responsive devices that vary compression ratios in engine cylinders.
2. Background Art
A xe2x80x9ccompression ratioxe2x80x9d of an internal combustion engine is defined as the ratio of the volume in a cylinder above a piston when the piston is at bottom-dead-center (BDC) to the volume in the cylinder above the piston when the piston is at top-dead-center (TDC). In general, increasing a compression ratio results in increasing the thermal efficiency of the engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine.
A known variable compression ratio system is disclosed in U.S. Pat. No. 4,834,031 entitled xe2x80x9cVariable Compression-Ratio Control Device For An Internal Combustion Engine.xe2x80x9dThe known system utilizes a variable compression ratio (VCR) mechanism for varying a compression ratio of an engine cylinder between a high compression ratio and a low compression ratio. The system also utilizes a sensor for detecting a current compression ratio and a fault-detecting unit for detecting a malfunction of the sensor. When a fault is detected in the sensor, the system lowers the compression ratio of the engine cylinder.
The known system, however, does not recognize nor solve the problem of controlling an engine when one or more VCR mechanisms are unable to transition the corresponding engine cylinders from a high compression ratio to a low compression ratio. Thus, when one or more of cylinders are not transitioned to the low compression ratio and the remaining cylinders are transitioned to the low compression ratio, the engine cylinders are not all be operating at the same compression ratio. Thus, the engine cylinders at a high compression ratio may produce higher torques than the engine cylinders at the lower compression ratio, resulting in undesirable engine torque fluctuations.
A system and a method are provided that addresses the problems of the known system by reducing and/or eliminating engine torque fluctuations when at least one VCR mechanism in an engine is unable to lower a compression ratio in a corresponding engine cylinder.
A method is provided that controls an engine having first and second pressure responsive devices that can vary compression ratios in first and second engine cylinders, respectively. A pressure responsive device is defined as any device capable of varying a compression ratio in an engine cylinder responsive to a received fluid pressure or pressure signal. For example, a pressure responsive device may include one or more of the following devices: (i) a variable length connecting rod as taught herein, (ii) a variable length connecting rod, such as a connecting rod using an eccentric ring disposed about a crankpin, as disclosed in U.S. Pat. No. 5,562,068, (iii) a variable length connecting rod having a gear arrangement for varying the length of the connecting rod, (iv) a device that can vary a position of a cylinder relative to a crankshaft, (v) a device that can vary an internal geometry or volume of an engine cylinder, (vi) a device that can vary a piston geometry, such as a piston height or thickness for example, and (vii) a crankshaft having an eccentric crankshaft bearing where the operating position of the bearing is varied to vary a cylinder compression ratio.
The method includes commanding the first and second devices to decrease compression ratios in the first and second cylinders, respectively. The method further includes indicating when the first device has not decreased a compression ratio in said first cylinder. Finally, the method includes commanding said second device to increase a compression ratio in said second cylinder to reduce engine torque fluctuations.
A system is provided for controlling an engine having first and second pressure responsive devices varying compression ratios in first and second engine cylinders, respectively. The system includes a sensor that generates a signal indicative of a compression ratio of the first cylinder. The sensor may comprise a (i) positional sensor that indicates an effective length of a variable length connecting rod that is indicative of a compression ratio in the first cylinder, or (ii) a pressure sensor that measures pressures in said first cylinder at a piston top-dead-center (TDC) position and the piston bottom-dead-center (BDC) position indicative of the compression ratio in the first cylinder. The system further includes a controller operably connected to the sensor. The controller is configured to command the first and second devices to reduce compression ratios in the first and second cylinders, respectively, from a first compression ratio to a second compression ratio. The controller is further configured to determine when the first device has not changed a compression ratio in the first cylinder. Finally, the controller is configured to command the second device to obtain the first compression ratio in the first cylinder to reduce engine torque fluctuations.
The control system and method for controlling an engine in accordance with the present invention provides a substantial advantage over known systems and methods. The system and method reduces undesirable engine torque fluctuations when one or more variable compression engine cylinders are unable to transition to a low compression ratio. In particular, when this malfunction occurs, the system and method transitions all of the engine cylinders to a high compression ratio to obtain an equivalent torque output in the engine cylinders to reduce engine torque fluctuations.